PMDC motors are well-known and have many uses. The current trend, driven by customer demand, is for smaller, lighter and/or more powerful motors. One such application is for the fan motor of a cooling module for a vehicle engine. Space is always a premium in the modern vehicle engine compartment and weight is a prime target for reducing fuel consumption. Hence the desire for a smaller, lighter motor, without a reduction in performance.
Increasing the number of stator poles say from two poles to four poles, increases the power density, which is important when trying to increase the performance of the motor. However, other factors are also affected. Normally when reducing the physical size of a motor, the performance will be downgraded, if no other design changes are made.
Wound rotors with commutators are commonly used in PMDC (permanent magnet direct current), motors. This type of rotor has a number of salient poles or teeth about which the armature windings are wound. The windings form coils about the teeth and the rotor has a number of slots which form winding tunnels through which the windings extend. The coils thus have two axially extending portions which lie within the slots and two end portions, known as heads, which extend transversely to the axial direction of either axial end of the rotor core. The heads provide no force and serve only to provide electrical connections between the axially extending portions of the coil. In most rotors, some of the heads cover or cross over other heads due to the arrangement of the coils and the winding technique. Thus, some heads must extend axially out from the rotor core a considerable distance in order to cross one, two or more preciously wound heads. This distance adds nothing to the motor performance but does add to the axial length of the motor and of course, to the total amount of wire used to form the coils.
One known way of reducing the head height is described in European Patent No. EP 1093208 by Gate SPA shows a four pole, twenty slot motor having an armature lamination known as an asymmetric lamination. The asymmetric lamination was designed to reduce the winding head height by reducing or eliminating winding head overlap. However, this design proves to be difficult to successfully put into production due to the complex slot shapes.
Another way is to use a winding technique known as concentrated winding. A concentrated winding is a winding which is wound around a single pole. In this way, the head height is kept to a minimum, because no coils overlap coils of another pole. However, for an armature with many rotor poles, concentrated windings are usually not a option especially for miniature and small sized PMDC motors.
As the rotor poles are produced by making slots in the rotor core, the rotor pole number is often referred to as the rotor slot number, or simply as the slot number to reduce confusion with the stator pole number, which is then referred to simply as the pole number. Commutator segments are commonly referred to as bars. Thus a motor with 6 stator poles, 9 rotor poles and a commutator with 18 segments is referred to as a 6 pole 9 slots 18 bar motor. This convention will be used in this specification, where appropriate. When the number of bars equals the number of slots, it is usual to not mention the number of bars. To avoid confusion between stator poles and rotor poles, the terms tooth and teeth may be used when referring to the rotor poles.